1. Field of the Invention
The present invention relates to a monoclonal antibody or an antibody fragment thereof, which binds to human CD4 extracellular region with a high affinity and also exhibits a high antibody-dependent cellular cytotoxicity (hereinafter, referred to as “ADCC activity”) or a high complement-dependent cellular cytotoxicity (hereinafter, referred to as “CDC activity”); a hybridoma which produces the antibody; a DNA which encodes the antibody; a vector which contains the DNA; a transformant obtainable by introducing the vector; a process for producing an antibody or an antibody fragment thereof using the hybridoma or the transformant; and a therapeutic agent and a diagnostic agent using the antibody or the antibody fragment thereof.
2. Brief Description of the Background Art
Cluster of differentiation 4 (hereinafter, referred to as “CD4”) is a glycoprotein having a molecular weight of about 55 kDa, which is expressed on the cell surface of most of thymic cells, about ⅔ of peripheral blood T cells, monocytes, and macrophage. CD4 is a type I transmembrane protein in which four immunoglobulin superfamily domains (designated in order as D1 to D4 from the N terminal to the cell membrane side) are present on the outside of the cells, and two N-linked sugar chains in total are bound to the domains D3 to D4. CD4 binds to a major histocompatibility complex (MHC) class II molecule through D1 and D2 domains, and then activates the T cells. Further, it is also known that CD4 polymerizes through D3 and D4 domains. D1 domain of CD4 is known to serve as a receptor for a human immunodeficiency virus (hereinafter, referred to as “HIV”) (Non-Patent Literature 1).
CD4 is also known as T4, and the gene has been cloned in 1985 (Non-Patent Literature 2), and the DNA sequence, the amino acid sequence and the three-dimensional structure of CD4 are publicly available from a known database. For example, these can be obtained by reference to Accession Nos. P01730 (SWISSPROT), M12807 (EMBL), and the like.
The anti-CD4 monoclonal antibody OKT4 was first confirmed as a monoclonal antibody (hereinafter, referred to as “mAb”) which binds to CD4 (Non-Patent Literature 3). Since then, a large number of monoclonal antibodies against CD4 (hereinafter, referred to as “anti-CD4 mAbs”) have been reported. Most of anti-CD4 mAbs reported hitherto are known to recognize D1 domain (Non-Patent Literature 1). Some of anti-CD4 mAbs are under clinical development for the purpose of treating cancers, immune diseases, and infections. For example, based on the fact that the binding between CD4 and HIV is essential for the infection of HIV, an antibody which recognizes D1 domain of CD4 can inhibit the infection of HIV, under the development as an HIV therapeutic agent.
Examples of the anti-CD4 mAb developed as a therapeutic agent for cancers or immune diseases include zanolimumab (6G5) (Non-Patent Literature 4), keliximab (CE9.1) (Non-Patent Literature 5), and the like. These antibodies are antibodies which exert their medicinal efficacy by specifically attacking CD4-expressing cells which are target cells, and it is considered that the mechanism of medicinal efficacy is mainly due to an ADCC activity (Non-Patent Literatures 6 and 7). Meanwhile, these antibodies are shown to be devoid of a CDC activity which is generally known as one of the main mechanism of medicinal efficacy of therapeutic antibodies, like ADCC (Non-Patent Literatures 6 and 7).
The potency of CDC activity is different depending on the subclass of antibodies, and human IgG1 and IgG3 subclasses have a high CDC activity. It is known that the intensity of CDC activity among the subclasses is generally in the order of IgG3≧IgG1>>IgG2≈IgG4. In addition, there is a case where the CDC activity of an antibody is exerted or is not exerted depending on an antigen to which the antibody binds (Non-Patent Literature 11). Accordingly, every antibody cannot always exhibit the CDC activity.
Among the anti-CD4 mAbs confirmed hitherto, little is known of the antibodies having a CDC activity. Uniquely, even though it is known that OKT4 exhibits a CDC activity against a CD4-positive human cell line only when a rabbit serum which is high in complement number is used (Non-Patent Literature 8), no examples of an antibody which exhibits a CDC activity against a CD4-positive human cell line when a human complement is used have been reported.
A dissociation constant KD of an antibody for several known anti-CD4 mAbs has been reported. For example, antibodies shown in the following table have been reported to have a dissociation constant KD of about 7 to 0.01 nM (Table 1). Upon the calculation of dissociation constants, there is a possibility that measurement values vary depending on assay equipments, assay methods, and analysis methods. Therefore, when comparing dissociation constants, the values which were measured and analyzed under the same conditions are required.
TABLE 1Clone NameDissociation Constant [M]6G5*7.1 × 10−9 CE9.1*3.2 × 10−11Leu-3a**1.0 × 10−1113B8.2***5.0 × 10−9 *WO97/13852,**Non-Patent Literature 9,***Non-Patent Literature 10    Non-Patent Literature 1: Leucocyte Typing VI, 49 (1997)    Non-Patent Literature 2: Cell, 42, 93 (1985)    Non-Patent Literature 3: Proc. Natl. Acad. Sci. USA, 76, 4061 (1979)    Non-Patent Literature 4: Blood, 109, 4655 (2007)    Non-Patent Literature 5: Lancet, 352, 1109 (1998)    Non-Patent Literature 6: Clin Immunol Immunopathol, 84, 73 (1997)    Non-Patent Literature 7: Cancer Res, 67, 9945 (2007)    Non-Patent Literature 8: J Immunol, 164, 1925 (2000)    Non-Patent Literature 9: J Immunol, 145, 2839 (1990)    Non-Patent Literature 10: Clinical Immunology, 119, 38 (2006)    Non-Patent Literature 11: J. Immunol., 174, 5706 (2005)